The present invention relates generally to security tags used with electronic security systems for detecting the presence or unauthorized removal of articles and, more particularly, to such security tags which are particularly adapted for use with articles having an inherent capacitance.
The use of electronic security systems for detecting and preventing theft or other unauthorized removal of articles or goods from retail establishments and/or other facilities, such as libraries, has become widespread. In general, such security systems employ a security tag which is associated with or is secured to an article (or its packaging) which is readily accessible to potential customers or facility users and, therefore, may be easily removed. Security tags may take on many different sizes, shapes and forms, depending upon the particular type of security system in use, the type and size of the article, its packaging, etc. In general, such electronic security systems are employed for detecting the presence (or the absence) of a security tag and, thus, a protected article, as the protected article passes through or near a surveilled security area. In most cases, the surveilled area is located at or around an exit or entrance to a retail establishment or other facility.
One such electronic article security system which has gained widespread popularity utilizes a security tag which includes a self-contained, operatively tuned or resonant circuit in the form of a small, generally planar printed circuit which resonates at a predetermined frequency within a predetermined detection frequency range. A transmitter, which is tuned to a frequency within the detection frequency range, is employed for transmitting electromagnetic energy into the surveilled area. A receiver, also tuned to the detection frequency range, is positioned generally proximate to the surveilled area. Typically, the transmitter is located on one side of an exit and the receiver is located on the other side of the exit so that a person must pass between the transmitter and the receiver in order to exit the facility. In this manner, when an article having an attached security tag moves into or otherwise passes through the surveilled area, the tag is exposed to the transmitted energy. Upon receiving the transmitted energy, the resonant circuit of the tag resonates, providing an output signal detectable by the receiver. When the receiver detects such an output signal, indicative of the presence of an article with a security tag within the surveilled area, the receiver activates an alarm to alert appropriate security or other personnel.
Electronic article security systems of the type discussed above have been shown to be very effective in preventing the theft or unauthorized removal of articles, particularly articles which are relatively high in value and relatively small in size, such as jewelry, expensive clothing, video cassettes, etc. While such electronic article security systems have been generally effective, there are certain articles having properties which have made it difficult to effectively use such electronic article security systems. For example, it has been found that certain articles, such as articles containing liquid, or certain organic articles, such as meat or living animals (including humans), possess inherent electrical properties, usually inherent capacitance. When a security tag is attached to such an article, the inherent capacitance of the article interacts with the resonant circuit of the tag generally adding both capacitance and resistance to the resonant circuit. The addition of capacitance and resistance to the resonant circuit affects both the resonant frequency of the circuit and the Q factor of the circuit. It has been determined that in a typical standard security tag having an inductance/capacitance resonant circuit, attachment of the tag to an article having an inherent capacitance typically results in a shifting of the resonant frequency of the resonant circuit in the range of 500 to 1500 KHz and a reduction in the Q factor of the circuit in the range of from 40 to 80 percent. The precise shift in the resonant frequency of the circuit and the precise reduction in the Q factor depends upon many variables, including the particular material of the article and its electrical conductivity, the size of the article, the proximity of the tag to the article, the temperature of the article, etc. However, such a shift in the resonant frequency when coupled with such a reduced Q factor, in the case of some articles, is sufficient to so diminish the performance of the resonant circuit as to materially diminish the effectiveness of the electronic article security system for use in detecting the unauthorized removal or presence of such articles.
The present invention relates to an article compensating security tag which is particularly adapted for use with an article having inherent electrical properties, such as inherent capacitance, which interact with the circuitry of the security tag. The security tag of the present invention is particularly well suited for protecting meat, particularly expensive cuts of meat. A security tag made in accordance with the present invention is designed to provide an initial resonant frequency which is above the center frequency of the article security system so that when the tag is attached to an article having such inherent capacitance, the inherent capacitance interacts with the resonant circuit to shift the resonant frequency of the resonant circuit closer to the center frequency of the security system.